Light-Emitting Element, Light-Emitting Device, Display Device, Electronic Appliance, and Lighting Device

ABSTRACT

A multicolor light-emitting element using fluorescence and phosphorescence, which has a small number of manufacturing steps owing to a relatively small number of layers to be formed and is advantageous for practical application can be provided. In addition, a multicolor light-emitting element using fluorescence and phosphorescence, which has favorable emission efficiency is provided. A light-emitting element which includes a light-emitting layer having a stacked-layer structure of a first light-emitting layer exhibiting light emission from a first exciplex and a second light-emitting layer exhibiting phosphorescence is provided.

This application is a continuation of copending U.S. application Ser.No. 14/738,112, filed on Jun. 12, 2015 which is a continuation of U.S.application Ser. No. 13/955,904, filed on Jul. 31, 2013 (now U.S. Pat.No. 9,059,421 issued Jun. 16, 2015) which are all incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic appliance, and a lightingdevice each of which uses an organic compound as a light-emittingsubstance.

BACKGROUND ART

In recent years, research and development of a light-emitting element(organic EL element) which uses an organic compound and utilizeselectroluminescence (EL) have been actively promoted. In the basicstructure of such a light-emitting element, an organic compound layercontaining a light-emitting substance (an EL layer) is interposedbetween a pair of electrodes. By voltage application to this element,light emission from the light-emitting substance can be obtained.

Such a light-emitting element is a self-luminous element and hasadvantages over a liquid crystal display in having high pixel visibilityand eliminating the need for backlights, for example; thus, such alight-emitting element is thought to be suitable for a flat paneldisplay element. A display including such a light-emitting element isalso highly advantageous in that it can be thin and lightweight.Besides, very high speed response is one of the features of such anelement.

In such a light-emitting element, light-emitting layers can besuccessively formed two-dimensionally, so that planar light emission canbe obtained. Thus, a large-area element can be easily formed. Thisfeature is difficult to obtain with point light sources typified byincandescent lamps and LEDs or linear light sources typified byfluorescent lamps. Thus, the light-emitting element also has greatpotential as a planar light source which can be applied to a lightingdevice and the like.

In the case of such an organic EL element, electrons from a cathode andholes from an anode are injected into an EL layer, so that currentflows. By recombination of the injected electrons and holes, the organiccompound having a light-emitting property is put in an excited state toprovide light emission.

The excited state of an organic compound can be a singlet excited stateor a triplet excited state, and light emission from the singlet excitedstate (S*) is referred to as fluorescence, and light emission from thetriplet excited state (T*) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S*:T*=1:3.

In a compound that emits light from the singlet excited state(hereinafter, referred to as fluorescent substance), at roomtemperature, generally light emission from the triplet excited state(phosphorescence) is not observed while only light emission from thesinglet excited state (fluorescence) is observed. Therefore, theinternal quantum efficiency (the ratio of generated photons to injectedcarriers) of a light-emitting element using a fluorescent substance isassumed to have a theoretical limit of 25% based on the ratio of S* toT* which is 1:3.

In contrast, in a compound that emits light from the triplet excitedstate (hereinafter, referred to as phosphorescent compound), lightemission from the triplet excited state (phosphorescence) is observed.Further, since intersystem crossing (i.e., transfer from a singletexcited state to a triplet excited state) easily occurs in aphosphorescent compound, the internal quantum efficiency can beincreased to 100% in theory. That is, higher emission efficiency than inthe case of using a fluorescent substance can be achieved. For thesereasons, in order to achieve a highly efficient light-emitting element,a light-emitting element using a phosphorescent compound has beenactively developed recently.

A white light-emitting element disclosed in Patent Document 1 includes alight-emitting region containing plural kinds of light-emitting dopantswhich emit phosphorescence.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Translation of PCT International    Application No. 2004-522276

DISCLOSURE OF INVENTION

As a multicolor light-emitting element typified by a whitelight-emitting element, an element in which a layer in whichfluorescence is used as light with a short wavelength (fluorescentlayer), a layer in which phosphorescence is used as light with a longwavelength (phosphorescent layer), and an intermediate layer (chargegeneration layer) between the fluorescent layer and the phosphorescentlayer are provided has been developed and partly put into practicalapplication.

This structure, in which fluorescence is used as light with a shortwavelength and a problem in the lifetime and phosphorescence is used aslight with a long wavelength, is characterized by enabling a multicolorlight-emitting element with stable characteristics to be achieved inspite of lower efficiency than an element in which phosphorescence isused as light with both short and long wavelengths.

The multicolor light-emitting element having the above-describedstructure in which the reliability is put ahead of the performance issuitable for practical application; on the other hand, a larger numberof films are formed in order to obtain one light-emitting element, whichhinders the practical application of the light-emitting element.

In such an element, the intermediate layer is provided between thephosphorescent layer and the fluorescent layer and elements areconnected in series in order to prevent quenching of phosphorescence dueto the fluorescent layer.

For a fluorescent layer, a substance having a condensed aromatic ringskeleton, typified by anthracene, is generally used as a host material.Such a substance having a condensed aromatic ring skeleton has a lowtriplet level. Thus, in the case where the fluorescent layer is formedin contact with a phosphorescent layer, the triplet excited energygenerated in the phosphorescent layer is transferred to the tripletlevel of the host material in the fluorescent layer to be quenched.Meanwhile, the above-described problem is alleviated by using a hostmaterial with high triple excited energy for the fluorescent layer. Inthat case, however, the singlet excited energy of the host material isincreased (becomes too high), so that energy is not sufficientlytransferred from the host material to a fluorescent dopant, resulting ininsufficient emission efficiency in the fluorescent layer. As a result,a nonradiative deactivation process of the host material might beincreased to degrade the characteristics (particularly, lifetime) of theelement.

In view of the above, an object of one embodiment of the presentinvention is to provide a multicolor light-emitting element usingfluorescence and phosphorescence, which has a small number ofmanufacturing steps owing to a relatively small number of layers to beformed and which is advantageous for practical application.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element using fluorescence andphosphorescence, which has favorable emission efficiency.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element using fluorescence andphosphorescence, which has a relatively small number of layers to beformed, is advantageous for practical application, and has favorableemission efficiency.

Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic appliance, anda lighting device each of which can be manufactured at low cost by usingany of the above-described light-emitting elements.

Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic appliance, anda lighting device each of which has reduced power consumption by usingany of the above-described light-emitting elements.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

Any of the above-described objects can be achieved by a light-emittingelement which includes a light-emitting layer having a stacked-layerstructure of a first light-emitting layer exhibiting light emission froma first exciplex and a second light-emitting layer exhibitingphosphorescence. It is preferable that light emitted from the firstlight-emitting layer have an emission peak on the shorter wavelengthside than light emitted from the second light-emitting layer.

An exciplex is an excited state formed from two kinds of substances. Inthe case of photoexcitation, the exciplex is formed in such a mannerthat one molecule in an excited state takes in the other substance in aground state. Thus, when the exciplex emits light to be in a groundstate, it returns to be the original substances. For this reason, aground state of the exciplex does not exist and energy transfer betweenthe exciplexes does not occur in principle. This makes it difficult forenergy transfer through a host material occupying a large part of thelight-emitting layer to occur, and makes it easy for the light-emittingelement having the structure to achieve both fluorescence from a firstexciplex in the first light-emitting layer and phosphorescence from aphosphorescent compound in the second light-emitting layer.

The luminance of a light-emitting element using a phosphorescentsubstance emitting light with a short wavelength (e.g., substanceemitting blue phosphorescence) tends to degrade quickly. In view of theabove, fluorescence with a short wavelength is used, so that alight-emitting element with less degradation of luminance can beprovided. Note that in the light-emitting element of one embodiment ofthe present invention, the first light-emitting layer which is afluorescent layer and the second light-emitting layer which is aphosphorescent layer are stacked in contact with each other; thus, thelight-emitting element has a small number of layers included in an ELlayer, is advantageous in terms of cost, and is suitable for massproduction. Moreover, although the fluorescent layer and thephosphorescent layer are in contact with each other, deactivation of thetriplet excited level is less likely to occur owing to the use of theabove-described exciplex; thus, both phosphorescence and fluorescencecan be obtained.

One embodiment of the present invention is a light-emitting elementwhich includes a first electrode, a second electrode, and an EL layerinterposed between the first electrode and the second electrode. The ELlayer includes a light-emitting layer in which at least a firstlight-emitting layer and a second light-emitting layer are stacked. Thefirst light-emitting layer contains at least a first organic compoundand a second organic compound. The second light-emitting layer containsat least a third organic compound and a phosphorescent substance. Acombination of the first organic compound and the second organiccompound forms a first exciplex.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which light emission of thefirst exciplex has an emission peak on the shorter wavelength side thanlight emission of the phosphorescent substance.

The singlet excited level and the triplet excited level of the exciplexare close to each other. Thus, reverse intersystem crossing from thetriplet excited level of the exciplex to the singlet excited levelthereof easily occurs. In other words, delayed fluorescence is easilyemitted. The exciplex from which delayed fluorescence can be obtained isused for the first light-emitting layer, whereby the triplet excitedstate can also be converted into light emission; thus, thelight-emitting element can have higher emission efficiency than in thecase of using a normal fluorescent substance. For sufficient reverseintersystem crossing from the triplet excited level to the singletexcited level, a small difference in energy between the triplet excitedlevel and the singlet excited level is advantageous; the difference inenergy is preferably greater than or equal to 0 eV and less than orequal to 0.2 eV, more preferably greater than or equal to 0 eV and lessthan or equal to 0.1 eV.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which the firstexciplex efficiently exhibits reverse intersystem crossing from thetriplet excited level to the singlet excited level.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which the first exciplexexhibits delayed fluorescence.

It is preferable that in the second light-emitting layer, energy betransferred efficiently from a host to the phosphorescent compound.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which the secondlight-emitting layer further contains a fourth organic compound and acombination of the third organic compound and the fourth organiccompound formed is a second exciplex. Further, the lowest-energy-sideabsorption band of the phosphorescent substance and an emission spectrumof the second exciplex preferably overlap with each other.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which the difference inequivalent energy values between a peak wavelength of thelowest-energy-side absorption band of the phosphorescent substance and apeak wavelength of the emission spectrum of the second exciplex is lessthan or equal to 0.2 eV.

Although there is no limitation on each of the combination of the firstorganic compound and the second organic compound and the combination ofthe third organic compound and the fourth organic compound as long as anexciplex can be formed, one organic compound is preferably a materialhaving an electron-transport property and the other organic compound ispreferably a material having a hole-transport property. This structureallows efficient formation of an exciplex. Further, by changing themixture ratio, the transport property of the light-emitting layer itselfcan be adjusted and a recombination region can be easily adjusted. As aresult, extreme localization of the recombination region can beprevented, leading to a long lifetime of the element.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which one of the firstorganic compound and the second organic compound is a material having anelectron-transport property and the other is a material having ahole-transport property and one of the third organic compound and thefourth organic compound is a material having an electron-transportproperty and the other is a material having a hole-transport property.

In the light-emitting element of one embodiment of the presentinvention, each light-emitting layer is preferably formed of a materialhaving a hole-transport property and a material having anelectron-transport property as described above. The recombination regionin the light-emitting layer is preferably in the vicinity of aninterface between the first light-emitting layer and the secondlight-emitting layer. Thus, in the light-emitting element having theabove-described structure, one of the light-emitting layers, which iscloser to an anode than the other light-emitting layer, preferablycontains a large amount of material having a hole-transport property,and the other light-emitting layer, which is closer to a cathode thanthe one light-emitting layer, preferably contains a large amount ofmaterial having an electron-transport property. This structure allowsthe recombination region to be in the vicinity of the interface betweenthe first light-emitting layer and the second light-emitting layer,which is convenient for distribution of recombination energy.

Further, the first exciplex may be the same as the second exciplex.Thus, another embodiment of the present invention is a light-emittingelement in which the combination of the first organic compound and thesecond organic compound is the same as the combination of the thirdorganic compound and the fourth organic compound.

Part of a layer formed of the two kinds of substances forming theexciplex is doped with a phosphorescent substance, which enables amulticolor light-emitting element to be easily obtained. Note that inthis case, the ratio of the material having a hole-transport property tothe material having an electron-transport property may be differentbetween the first light-emitting layer and the second light-emittinglayer.

The emission spectrum of the light-emitting element having any of theabove-described structures is made by combining light emission from thefirst exciplex and light emission from the phosphorescent substance, andthus has at least two peaks.

Here, a fluorescent substance may be further contained as a dopant inthe first light-emitting layer. As described above, since the singletexcited level and the triplet excited level of the exciplex formed inthe first light-emitting layer are close to each other, reverseintersystem crossing from the triplet excited level to the singletexcited level easily occurs. That is, part of the triplet excited statecan be converted into the singlet excited state; thus, the proportion ofthe singlet excited state is higher than that in the conventional case(25%). Thus, the energy of the singlet excited state with the increasedproportion is transferred to the fluorescent substance, whereby thelight-emitting element can have higher emission efficiency than alight-emitting element using a normal fluorescent substance. Theadvantage of this structure is that the fluorescent substance with ahigh fluorescent quantum yield can be used as the dopant. As describedabove, a material having a function of generating the singlet excitedstate from the triplet excited state (the exciplex) is different from amaterial having a function of efficiently obtaining light emission fromthe singlet excited state (the fluorescent compound, which is a dopant),whereby high emission efficiency can be easily obtained. Further, sincethe light-emitting substance exists as the dopant, quenching or achemical reaction due to an impurity can be prevented, leading to a longlifetime.

When light emission colors which are complementary to each other areselected, the light-emitting element having any of the above-describedstructures can provide white light emission.

Another embodiment of the present invention is a light-emitting modulewhich includes the light-emitting element having any of theabove-described structures and a means which controls the light-emittingelement.

Another embodiment of the present invention is a display module whichincludes the light-emitting element having any of the above-describedstructures in a display portion and a means which controls thelight-emitting element.

Another embodiment of the present invention is a lighting device whichincludes the light-emitting element having any of the above-describedstructures.

Another embodiment of the present invention is a light-emitting devicewhich includes the light-emitting element having any of theabove-described structures and a means which controls the light-emittingelement.

Another embodiment of the present invention is a display device whichincludes the light-emitting element having any of the above-describedstructures in a display portion and a means which controls thelight-emitting element.

Another embodiment of the present invention is an electronic appliancewhich includes the light-emitting element having any of theabove-described structures.

The light-emitting device in this specification includes an imagedisplay device using a light-emitting element. Further, the category ofthe light-emitting device in this specification includes a module inwhich a light-emitting element is provided with a connector such as ananisotropic conductive film or a tape carrier package (TCP); a module inwhich the end of the TCP is provided with a printed wiring board; and amodule in which an IC (integrated circuit) is directly mounted on alight-emitting element by a COG (chip on glass) method. Furthermore, thecategory includes light-emitting devices which are used in lightingequipment or the like.

According to one embodiment of the present invention, a multicolorlight-emitting element using fluorescence and phosphorescence, which hasa relatively small number of layers to be formed and is advantageous forpractical application, can be provided.

According to another embodiment of the present invention, a multicolorlight-emitting element using fluorescence and phosphorescence, which hasfavorable emission efficiency, can be provided.

According to another embodiment of the present invention, a multicolorlight-emitting element using fluorescence and phosphorescence, which hasa relatively small number of layers to be formed, is advantageous forpractical application, and has favorable emission efficiency, can beprovided.

According to another embodiment of the present invention, alight-emitting device, a display device, an electronic appliance, and alighting device each of which can be manufactured at low cost by usingany of the above-described light-emitting elements can be provided.

According to another embodiment of the present invention, alight-emitting device, a display device, an electronic appliance, and alighting device each of which has reduced power consumption by using anyof the above-described light-emitting elements can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of light-emitting elements.

FIGS. 2A and 2B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 3A and 3B are conceptual diagrams of active matrix light-emittingdevices.

FIG. 4 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 5A and 5B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 6A and 6B illustrate a lighting device.

FIGS. 7A, 7B1, 7B2, 7C, and 7D illustrate electronic appliances.

FIG. 8 illustrates a light source device.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic appliance.

FIG. 13 shows luminance-current density characteristics of alight-emitting 1.

FIG. 14 shows current efficiency-luminance characteristics of thelight-emitting 1.

FIG. 15 shows luminance-voltage characteristics of the light-emittingelement 1.

FIG. 16 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 1.

FIG. 17 shows an emission spectrum of the light-emitting element 1.

FIG. 18 shows luminance-current density characteristics of alight-emitting element 2.

FIG. 19 shows current efficiency-luminance characteristics of thelight-emitting element 2.

FIG. 20 shows luminance-voltage characteristics of the light-emittingelement 2.

FIG. 21 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 2.

FIG. 22 shows an emission spectrum of the light-emitting element 2.

FIG. 23 shows luminance-current density characteristics of alight-emitting 3.

FIG. 24 shows current efficiency-luminance characteristics of thelight-emitting element 3.

FIG. 25 shows luminance-voltage characteristics of the light-emittingelement 3.

FIG. 26 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 3.

FIG. 27 shows an emission spectrum of the light-emitting element 3.

FIG. 28 shows luminance-current density characteristics of alight-emitting 4.

FIG. 29 shows current efficiency-luminance characteristics of thelight-emitting 4.

FIG. 30 shows luminance-voltage characteristics of the light-emittingelement 4.

FIG. 31 shows external quantum efficiency-luminance characteristics ofthe light-emitting 4.

FIG. 32 shows an emission spectrum of the light-emitting 4.

FIG. 33 shows time dependence of normalized luminance of thelight-emitting 4.

FIG. 34 shows emission spectra of single films of 2mDBTBPDBq-II andPCBNBB and a film formed by co-evaporation of 2mDBTBPDBq-II and PCBNBB.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. Note that the present inventionis not limited to the following description, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the invention should not be construedas being limited to the description in the embodiments below.

Embodiment 1

FIG. 1A is a diagram illustrating a light-emitting element of oneembodiment of the present invention. The light-emitting element includesat least a pair of electrodes (a first electrode 101 and a secondelectrode 102) and an EL layer 103 including a light-emitting layer 113.In the light-emitting layer 113, a first light-emitting layer 113 a anda second light-emitting layer 113 b are stacked in contact with eachother.

FIG. 1A also illustrates a hole-injection layer 111, a hole-transportlayer 112, an electron-transport layer 114, and an electron-injectionlayer 115 in the EL layer 103. However, this stacked-layer structure isan example, and the structure of the EL layer 103 in the light-emittingelement of one embodiment of the present invention is not limitedthereto. Note that in FIG. 1A, the first electrode 101 functions as ananode, and the second electrode 102 functions as a cathode.

The first light-emitting layer 113 a contains a first organic compoundand a second organic compound. The second light-emitting layer 113 bcontains a third organic compound and a phosphorescent compound. Thelight-emitting element of this embodiment is characterized in that acombination of the first organic compound and the second organiccompound forms a first exciplex. This structure enables fluorescenceoriginating from the first exciplex to be efficiently obtained from thefirst light-emitting layer, and phosphorescence originating from thephosphorescent compound to be efficiently obtained from the secondlight-emitting layer.

It is generally known that when a fluorescent layer and a phosphorescentlayer are stacked to obtain light emission, the triplet excited energyof a phosphorescent compound is transferred to a host material occupyinga large part of the fluorescent layer, resulting in a significantdecrease in emission efficiency. The reason is as follows: since asubstance having a condensed aromatic ring skeleton, typified byanthracene or the like, is generally used as a host material, the tripleexcited level is low and the triplet excited energy generated in thephosphorescent layer is transferred, resulting in nonradiative quenchingof the triplet excited energy. At present, it is difficult to obtain adesired emission wavelength and favorable element characteristics andreliability by using a substance other than the substance having acondensed aromatic ring skeleton; thus, it is difficult to obtain alight-emitting layer which includes a stack a fluorescent layer and aphosphorescent layer and has favorable characteristics.

The exciplex used in the first light-emitting layer of this embodimentis an excited state formed from two kinds of substances. In the case ofphotoexcitation, the exciplex is formed in such a manner that onemolecule in an excited state takes in the other substance in a groundstate. Thus, when the exciplex emits light to be in a ground state, itreturns to be the original substance. For this reason, a ground state ofthe exciplex does not exist, and energy transfer between the exciplexesor energy transfer from the other substance to the exciplex does notoccur in principle. This means that energy transfer to the firstexciplex does not occur in principle, so that in the light-emittingelement of this embodiment, energy transfer between the firstlight-emitting layer and the second light-emitting layer is controlledand both fluorescence and phosphorescence can be easily obtained.

When compounds whose triplet excited levels are higher than that of thephosphorescent compound (or the phosphorescent compound and the thirdorganic compound, or the phosphorescent compound, the third organiccompound, and the fourth organic compound) in the second light-emittinglayer 113 b are used as the first organic compound and the secondorganic compound in the first light-emitting layer 113 a, energytransfer from the triplet excited level of the phosphorescent compound(or the phosphorescent compound and the third organic compound, or thephosphorescent compound, the third organic compound, and the fourthorganic compound) to one or both of the first organic compound and thesecond organic compound in a ground state in the first light-emittinglayer 113 a can also be controlled.

Organic compounds which are brought into an excited state quickly forman exciplex; thus, energy transfer from organic compounds in excitedstates, which have not yet formed an exciplex, is less likely to occur.

From this, in the light-emitting element of this embodiment, energytransfer through a host material occupying a large part of thelight-emitting layer is less likely to occur, and both fluorescence ofthe first exciplex in the first light-emitting layer and phosphorescenceof the phosphorescent compound in the second light-emitting layer can beeasily obtained.

The first exciplex is formed from the first organic compound and thesecond organic compound to obtain light emission from the exciplex,whereby fluorescence with a desired wavelength can be obtained in spiteof the use of the substance with a high triplet excited level. Further,since light emission from the exciplex originates from a difference inenergy between the shallower HOMO level of the HOMO levels of the twocompounds and the deeper LUMO level of the LUMO levels of the twocompounds, desired light emission can be easily obtained by changing thecombination of the first organic compound and the second organiccompound.

A fluorescent substance may be further contained as a dopant in thefirst light-emitting layer 113 a up to several weight percent(specifically, 0.01 wt % to 5 wt %, preferably 0.01 wt % to 1 wt %). Theabove-described amount does not cause energy transfer through thefluorescent substance, and energy transfer through a host material iscontrolled by the exciplex; thus, both fluorescence from the fluorescentsubstance and phosphorescence from the phosphorescent compound can beachieved. Further, the energy is transferred from the first exciplex tothe fluorescent substance with a high fluorescent quantum yield, wherebythe emission efficiency can be improved. As described above, since thesinglet excited level and the triplet excited level of the exciplexformed in the first light-emitting layer are close to each other,reverse intersystem crossing from the triplet excited level to thesinglet excited level easily occurs. That is, part of the tripletexcited state can be converted into the singlet excited state; thus, theproportion of the singlet excited state is higher than that in theconventional case (25%). Thus, the energy of the singlet excited statewith the increased proportion is transferred to the fluorescentsubstance, whereby the light-emitting element can have higher emissionefficiency than a light-emitting element using a normal fluorescentsubstance. As described above, a material having a function ofgenerating the singlet excited state from the triplet excited state (theexciplex) is different from a material having a function of efficientlyobtaining light emission from the singlet excited state (the fluorescentcompound, which is a dopant), whereby high emission efficiency can beeasily obtained. Further, since the light-emitting substance is used asthe dopant, quenching or a chemical reaction due to an impurity can beprevented, leading to a long lifetime.

Note that a fourth organic compound may be further contained in thesecond light-emitting layer 113 b so that the combination of the thirdorganic compound and the fourth organic compound form a second exciplex.Such a structure of the second light-emitting layer 113 b enables energytransfer between the light-emitting layers to be further controlled. Inaddition, as described later, energy transfer from the second exciplexto the phosphorescent compound can be enhanced.

Although there is no limitation on the combination of the first organiccompound and the second organic compound and the combination of thethird organic compound and the fourth organic compound as long as anexciplex can be formed, one organic compound is preferably a materialhaving a hole-transport property and the other organic compound ispreferably a material having an electron-transport property. This isbecause in this case, a donor-acceptor excited state is easily formed,which allows an exciplex to be efficiently formed. In the case where thecombination of the first organic compound and the second organiccompound and the combination of the third organic compound and thefourth organic compound are each a combination of the material having ahole-transport property and the material having an electron-transportproperty, the carrier balance can be easily controlled depending on themixture ratio. Specifically, the ratio of the material having ahole-transport property to the material having an electron-transportproperty is preferably 1:9 to 9:1.

Since the carrier balance can be easily controlled in the light-emittingelement having the above-described structure, a recombination region canalso be easily adjusted. In the light-emitting element of thisembodiment, the recombination region is preferably formed in thevicinity of an interface between the first light-emitting layer and thesecond light-emitting layer. As described above, the light-emittingelement of this embodiment has a structure capable of controlling energytransfer between the light-emitting layers; thus, the recombinationregion formed in the vicinity of the interface between the firstlight-emitting layer and the second light-emitting layer enables excitedenergy to be distributed to both of the light-emitting layers in abalanced manner. In terms of an even distribution of the excited energyto these light-emitting layers, the recombination region is morepreferably formed in the vicinity of the interface between the twolight-emitting layers. In order to form the recombination region ofcarriers in the vicinity of the interface between the firstlight-emitting layer 113 a and the second light-emitting layer 113 b,one of the first light-emitting layer 113 a and the secondlight-emitting layer 113 b, which is closer to the anode than the otherlight-emitting layer, is made to be a hole-transport layer, and theother, which is closer to the cathode than the one light-emitting layer,is made to be an electron-transport layer. Note that the recombinationregion can be adjusted easily by adjusting the ratio of the materialhaving a hole-transport property to the material having anelectron-transport property. The layer may contain a large amount ofmaterial having a hole-transport property to be a hole-transport layer;the layer may contain a large amount of material having anelectron-transport property to be an electron-transport layer. Note thatthe emission color can be easily adjusted by adjusting the ratio of thematerial having a hole-transport property to the material having anelectron-transport property.

The combination of the first organic compound and the second organiccompound may be the same as or different from the combination of thethird organic compound and the fourth organic compound. When thecombinations are the same (i.e., the first exciplex is the same as thesecond exciplex), energy transfer between the first light-emitting layerand the second light-emitting layer can be further controlled. Inaddition, since fewer kinds of materials are used, the light-emittingelement is advantageous in terms of cost and is put into practicalapplication more easily. Moreover, a carrier injection barrier at theinterface between the first light-emitting layer and the secondlight-emitting layer can be lowered, which contributes to a longlifetime of the element.

Needless to say, the first exciplex may be different from the secondexciplex. It is preferable to select the second exciplex having anemission wavelength matching an absorption wavelength of thephosphorescent compound as described later for higher emissionefficiency of the phosphorescent compound. Although there is highprobability that the first exciplex is different from the secondexciplex when such selection is made, energy transfer between theexciplexes does not occur in principle; thus, energy transfer betweenthe first light-emitting layer and the second light-emitting layer canbe suppressed, so that both fluorescence and phosphorescence can beeasily obtained. In addition, the light-emitting element can have higheremission efficiency.

Note that in the light-emitting element, light emitted from the firstlight-emitting layer preferably has a peak on the shorter wavelengthside than light emitted from the second light-emitting layer. Theluminance of a light-emitting element using the phosphorescent substanceemitting light with a short wavelength tends to degrade quickly. In viewof the above, fluorescence with a short wavelength is used, so that alight-emitting element with less degradation of luminance can beprovided. Note that in the light-emitting element of one embodiment ofthe present invention, the first light-emitting layer which is afluorescent layer and the second light-emitting layer which is aphosphorescent layer are stacked in contact with each other; thus, thelight-emitting element has a small number of layers included in an ELlayer, is advantageous in terms of cost, and is suitable for massproduction. Moreover, although the fluorescent layer and thephosphorescent layer are in contact with each other, deactivation of thetriplet excited level is less likely to occur owing to the use of theabove-described exciplex; thus, both phosphorescence and fluorescencecan be obtained.

Here, it is preferable that the first exciplex efficiently exhibitreverse intersystem crossing from the triplet excited level to thesinglet excited level. Since the exciplex is in a state with a smallenergy difference between the singlet excited state and the tripletexcited state, reverse intersystem crossing from the triplet excitedlevel to the singlet excited level easily occurs. In other words,delayed fluorescence is easily emitted. The use of the exciplex fromwhich delayed fluorescence is efficiently obtained for the firstlight-emitting layer enables also the triplet excited state to beconverted into light emission; thus, the light-emitting element can havehigher emission efficiency than in the case of using a normalfluorescent substance. Note that the delayed fluorescence here includesone exhibited and amplified by some heating (including self heatgeneration) (what is called thermally activated delayed fluorescence(TADF)). The delayed fluorescence is efficiently obtained under thecondition where the difference in energy between the triplet excitedlevel and the singlet excited level is greater than or equal to 0 eV andless than or equal to 0.2 eV, preferably greater than or equal to 0 eVand less than or equal to 0.1 eV. A structure using an exciplex havingthe above-described relation is preferable.

Further, in the light-emitting element of this embodiment, the firstlight-emitting layer and the second light-emitting layer are made toemit light with different emission wavelengths, so that thelight-emitting element can be a multicolor light-emitting element. Theemission spectrum of the light-emitting element is formed by combininglight having different emission peaks, and thus has at least two peaks.

Such a light-emitting element is suitable for obtaining white lightemission. When the first light-emitting layer and the secondlight-emitting layer emit light of complementary colors, white lightemission can be obtained. The white light-emitting element utilizesphosphorescence, has a smaller number of layers to be formed than astack-type light-emitting element, and can be provided at low cost inspite of having high emission efficiency. In addition, light emittedfrom the exciplex which efficiently exhibits delayed fluorescence isutilized as light with a short wavelength, whereby the light-emittingelement can have high emission efficiency and a long lifetime.

Here, for obtaining a light-emitting element having high emissionefficiency, energy transfer to the phosphorescent substance in thesecond light-emitting layer is considered. In this explanation, asubstance which supplies energy to the phosphorescent substance isreferred to as a host material. Carrier recombination occurs in both thehost material and the phosphorescent substance; thus, efficient energytransfer from the host material to the phosphorescent substance isneeded to increase emission efficiency. As mechanisms of the energytransfer from the host material to the phosphorescent substance, twomechanisms of Dexter mechanism and Förster mechanism have been proposed.

The efficiency of energy transfer from the host molecule to the guestmolecule (energy transfer efficiency Φ_(ET) is expressed by the formulagiven below. In the formula, k_(r) denotes a rate constant of alight-emission process (fluorescence in energy transfer from a singletexcited state, and phosphorescence in energy transfer from a tripletexcited state), k_(n) denotes a rate constant of a non-light-emissionprocess (thermal deactivation or intersystem crossing), and x denotes ameasured lifetime of an excited state.

$\begin{matrix}{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

First, according to the formula given above, it is understood that theenergy transfer efficiency φ_(ET) can be increased by significantlyincreasing the rate constant k_(h)*_(→g) of energy transfer as comparedwith another competing rate constant k_(r)+k_(n)(=1/τ). Then, in orderto increase the rate constant k_(h)*_(→g) of energy transfer, in Förstermechanism and Dexter mechanism, it is preferable that an emissionspectrum of a host molecule (a fluorescent spectrum in energy transferfrom a singlet excited state, and a phosphorescent spectrum in energytransfer from a triplet excited state) largely overlap with anabsorption spectrum of a guest molecule (a phosphorescent substance inthe second light-emitting layer).

Here, a longest-wavelength-side (lowest-energy-side) absorption band inthe absorption spectrum of the phosphorescent substance is important inconsidering the overlap between the emission spectrum of the hostmolecule and the absorption spectrum of the phosphorescent substance.

In an absorption spectrum of the phosphorescent substance, an absorptionband that is considered to contribute to light emission most greatly isat an absorption wavelength corresponding to direct transition from aground state to a triplet excitation state and a vicinity of theabsorption wavelength, which is on the longest wavelength side.Therefore, it is probably preferable that the emission spectrum (afluorescent spectrum and a phosphorescent spectrum) of the host materialoverlap with the absorption band on the longest wavelength side in theabsorption spectrum of the phosphorescent substance.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band at around 500 nm to 600mn as the absorption band on the longest wavelength side. Thisabsorption band is mainly based on a triplet MLCT (metal to ligandcharge transfer) transition. Note that the absorption band probably alsoincludes absorptions based on a triplet π-π* transition and a singletMLCT transition and these absorptions probably overlap each other toform a broad absorption band on the longest wavelength side in theabsorption spectrum. Therefore, as described above, it is preferablethat the broad absorption band on the longest wavelength side largelyoverlap with the emission spectrum of the host material when anorganometallic complex (especially iridium complex) is used as the guestmaterial.

Here, first, energy transfer from a host material in a triplet excitedstate will be considered. From the above-described discussion, it ispreferable that, in energy transfer from a triplet excited state, thephosphorescent spectrum of the host material and the absorption band onthe longest wavelength side of the phosphorescent substance largelyoverlap each other.

However, a question here is energy transfer from the host molecule inthe singlet excited state. In order to efficiently perform not onlyenergy transfer from the triplet excited state but also energy transferfrom the singlet excited state, it is clear from the above-describeddiscussion that the host material needs to be designed such that notonly its phosphorescent spectrum but also its fluorescent spectrumoverlaps with the absorption band on the longest wavelength side of theguest material. In other words, unless the host material is designed soas to have its fluorescent spectrum in a position similar to that of itsphosphorescent spectrum, it is not possible to achieve efficient energytransfer from the host material in both the singlet excited state andthe triplet excited state.

However, in general, the singlet excited level differs greatly from thetriplet excited level (singlet excited level>triplet excited level);therefore, the fluorescence emission wavelength also differs greatlyfrom the phosphorescence emission wavelength (fluorescence emissionwavelength<phosphorescence emission wavelength). For example,4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is commonlyused as a host molecule in a light-emitting element including aphosphorescent substance, has a phosphorescent spectrum at around 500 nmand has a fluorescent spectrum at around 400 nm, which are largelydifferent by about 100 nm. This example also shows that it is extremelydifficult to design a host material so as to have its fluorescentspectrum in a position similar to that of its phosphorescent spectrum.

Further, the energy level of the singlet excited level of a certainsubstance is higher than that of the triplet excited level; thus, thetriplet excited level of a host material whose fluorescence spectrumcorresponds to a wavelength close to an absorption spectrum of a guestmaterial on the longest wavelength side is lower than the tripletexcited level of the guest material.

However, an exciplex is used as a host material in the secondlight-emitting layer of the light-emitting element of this embodiment.Fluorescence from the exciplex has a spectrum on the longer wavelengthside than a fluorescence spectrum of the third organic compound alone orthe fourth organic compound alone. Therefore, energy transfer from asinglet excited state can be maximized while the triplet excited levelsof the third organic compound alone and the fourth organic compoundalone are kept higher than the triplet excited level of the guestmaterial. In addition, the exciplex is in a state where the tripletexcited level and the singlet excited level are close to each other;therefore, the fluorescence spectrum and the phosphorescence spectrumexist at substantially the same position. Accordingly, both thefluorescence spectrum and the phosphorescence spectrum of the exciplexcan overlap largely with an absorption corresponding to transition ofthe guest molecule from the singlet ground state to the triplet excitedstate (a broad absorption band in an absorption spectrum of the guestmolecule, which exists on the longest wavelength side); thus, thelight-emitting element can have high energy transfer efficiency.

As described above, the lowest-energy-side absorption band of thephosphorescent substance in the second light-emitting layer overlapswith the emission spectrum of the second exciplex in the secondlight-emitting layer, whereby the light-emitting element can have morefavorable emission efficiency. Further, the difference in equivalentenergy values between peak wavelengths of the lowest-energy-sideabsorption band of the phosphorescent substance and the emissionspectrum of the second exciplex is preferably less than or equal to 0.2eV, in which case the overlap between the absorption band and theemission spectrum is large.

In FIG. 1A, the first light-emitting layer 113 a is formed on the sidewhere the first electrode 101 functioning as the anode is formed and thesecond light-emitting layer 113 b is formed on the side where the secondelectrode 102 functioning as the cathode is formed. However, thestacking order may be reversed.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 is described below withreference to FIG. 1A.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 provided between thefirst electrode 101 and the second electrode 102. Note that in thisembodiment, the first electrode 101 functions as an anode and the secondelectrode 102 functions as a cathode. In other words, when voltage isapplied between the first electrode 101 and the second electrode 102 sothat the potential of the first electrode 101 is higher than that of thesecond electrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specific examples are indium oxide-tin oxide (ITO: indium tinoxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like. Such conductive metal oxide films areusually formed by a sputtering method, but may also be formed byapplication of a sol-gel method or the like. In an example of theformation method, indium oxide-zinc oxide is deposited by a sputteringmethod using a target obtained by adding 1 wt % to 20 wt % of zinc oxideto indium oxide. Further, a film of indium oxide containing tungstenoxide and zinc oxide (IWZO) can be formed by a sputtering method using atarget in which tungsten oxide and zinc oxide are added to indium oxideat 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively. In addition,gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), anitride of a metal material (such as titanium nitride), or the like canbe used. Graphene can also be used. Note that when a composite materialdescribed later is used for a layer which is in contact with the firstelectrode 101 in the EL layer 103, an electrode material can be selectedregardless of its work function.

There is no particular limitation on the stacked structure of the ELlayer 103 as long as the light-emitting layer 113 has the structuredescribed in Embodiment 1. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich a hole-injection layer 111, a hole-transport layer 112, alight-emitting layer 113, an electron-transport layer 114, and anelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Materials for the layers are specifically given below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be fanned using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) or N,N-′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS), or thelike.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to faunan electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the acceptorsubstance, 7, 7,8, 8-tetracyano -2,3 ,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), chloranil, and the like can be given. Inaddition, a transition metal oxide can be given. In addition, oxides ofmetals belonging to Group 4 to Group 8 of the periodic table can begiven. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

As the substance having a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher is preferably used. Organic compounds which can be used as thesubstance having a hole-transport property in the composite material arespecifically given below.

Examples of the aromatic amine compound includeN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB), N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

As carbazole derivatives which can be used for the composite material,the following can be given specifically:3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2);3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like.

In addition, examples of the carbazole derivatives which can be used forthe composite material include 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon which can be used for the compositematerial include 2-tert-butyl-9,10-di(2-naphthypanthracene(abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis (3 ,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyflanthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-l-naphthypanthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Besides, pentacene, coronene, or the like can also be used. As thesearomatic hydrocarbons given here, it is preferable that an aromatichydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or more and having14 to 42 carbon atoms be used.

The aromatic hydrocarbon which can be used for the composite materialmay have a vinyl skeleton. Examples of the aromatic hydrocarbon having avinyl group include 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 is a layer containing a substance having ahole-transport property. Examples of the substance having ahole-transport property include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yptriphenylamine(abbreviation: BPAFLP), and the like. The substances given here havehigh hole-transport properties and are mainly ones having a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the substance having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used. Note that the layer containing a substance having ahole-transport property is not limited to a single layer, and may be astack of two or more layers containing any of the above substances.

The light-emitting layer 113 has the structure of the light-emittinglayer 113, which is described in Embodiment 1. In other words, the firstlight-emitting layer 113 a and the second light-emitting layer 113 b arestacked in this order over the first electrode. The first light-emittinglayer 113 a contains a first organic compound and a second organiccompound. The second light-emitting layer 113 b contains a third organiccompound, a fourth organic compound, and a phosphorescent compound. Thelight-emitting element of this embodiment is characterized in that thecombination of the first organic compound and the second organiccompound forms a first exciplex and the combination of the third organiccompound and the fourth organic compound forms a second exciplex. Thefirst exciplex exhibits fluorescence and the second exciplex providesenergy for the phosphorescent compound, so that both fluorescence andphosphorescence can be efficiently obtained from the firstlight-emitting layer and the second light-emitting layer, respectively.

Examples of materials which can be used as the phosphorescent compoundin the second light-emitting layer 113 b are given below.

The examples include an organometallic iridium complex having a4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(Mptz1-mp)₃) ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃), ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such as bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium (III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²1iridium(III) picolinate(abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C²′}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation: FIr(acac)). These are compounds emittingblue phosphorescence and have an emission peak at 440 nm to 520 nm.

Other examples include organometallic iridium complexes havingpyrimidine skeletons, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)],(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)), (acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium (III) (abbreviation:Ir(nbppm)₂(acac)), (acetylacetonato)b is[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes havingpyrazine skeletons, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexeshaving pyridine skeletons, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such astris(acetylacetonato) (monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). These are mainly compounds emitting greenphosphorescence and have an emission peak at 500 nm to 600 nm. Note thatan organometallic iridium complex having a pyrimidine skeleton hasdistinctively high reliability and emission efficiency and thus isespecially preferable.

Other examples include(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), andbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(dlnpm)₂(dpm)); organometallic iridium complexes havingpyrazine skeletons, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)), or(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes havingpyridine skeletons, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) andbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂acac); a platinum complex such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum (II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). These are compounds emitting redphosphorescence and have an emission peak at 600 mn to 700 nm. Further,the organometallic iridium complex having a pyrazine skeleton canprovide red light emission with favorable chromaticity.

Known phosphorescent materials, other than the phosphorescent compoundsgiven above, may be selected and used.

Further, in the case where a fluorescent substance is contained in thefirst light-emitting layer 113 a, known fluorescent substances may beused other than compounds given below.

Examples of the fluorescent substance include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N'-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn),N,N-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryptriphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1¹-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N, 9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene propanedinitrile (abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), 2-(2,6-bis 244-(dimethylamino)phenyl]ethenyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: B isDCM), and2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation:BisDCJTM). Condensed aromatic diamine compounds typified bypyrenediamine compounds such as 1,6FLPAPrn and 1,6mMemFLPAPrn areparticularly preferable because of their high hole-trapping properties,high emission efficiency, and high reliability.

A substance efficiently exhibiting delayed fluorescence is included inthe examples of the fluorescent substance. The substance exhibitingdelayed fluorescence converts part of the triplet excited state into thesinglet excited state to contribute to light emission, so that emissionefficiency can be improved. That is, synergy effects between the use ofthe fluorescent substance and the formation of the exciplex in the firstlight-emitting layer can be expected. As such a material, materialsgiven below can be used.

A fullerene, a derivative thereof, an acridine derivative such asproflavine, eosin, or the like can be used. A metal-containing porphyrinsuch as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd),tin (Sn), platinum (Pt), indium (In), or palladium (Pd) can be given.Examples of the metal-containing porphyrin include a protoporphyrin-tinfluoride complex (SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex(SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF₂(HematoIX)), a coproporphyrin tetramethyl ester-tin fluoride complex(SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex(SnF₂(OEP)), an etioporphyrin-tin fluoride complex (SnF₂(Etio I)), andan octaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which areshown in the following structural formulae.

Alternatively, as the material exhibiting thermally activated delayedfluorescence, a heterocyclic compound including a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-α]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ), which is shown in the structural formula givenbelow. The heterocyclic compound is preferably used because of theπ-electron rich heteroaromatic ring and the π-electron deficientheteroaromatic ring, for which the electron-transport property and thehole-transport property are high. Note that a substance in which theπ-electron rich heteroaromatic ring is directly bonded to the π-electrondeficient heteroaromatic ring is particularly preferably used becausethe donor property of the π-electron rich heteroaromatic ring and theacceptor property of the π-electron deficient heteroaromatic ring areboth increased and the energy difference between the S₁ level and the T₁level becomes small.

There is no particular limitation on the materials which can be used asthe first organic compound, the second organic compound, the thirdorganic compound, and the fourth organic compound as long as thecombination of the materials satisfies the conditions described inEmbodiment 1. A variety of kinds of carrier-transport materials can beselected.

Examples of the material having an electron-transport property include aheterocyclic compound having a polyazole skeleton, such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis [2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPB 0), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3 ,4-oxadi azo le(abbreviation: PBD),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation:OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazo le(abbreviation: CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPB I),or 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton, such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDB q-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo [f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), or4,6-bis [3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeleton,such as 2[3′-(dibenzothiophen-4-yObiphenyl-3-yl]dibenzo [fh]quino1ine(abbreviation: 2mDBTBPDB Qu-II), 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), or1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among theabove materials, a heterocyclic compound having a diazine skeleton and aheterocyclic compound having a pyridine skeleton have high reliabilityand are thus preferable. Specifically, a heterocyclic compound having adiazine (pyrimidine or pyrazine) skeleton has a high electron-transportproperty to contribute to a reduction in drive voltage.

Examples of the material having a hole-transport property include acompound having an aromatic amine skeleton, such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenyl amine(abbreviation: PCBNBB), 9,9-dimethyl-N-phenyl-N4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton, suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties,to contribute to a reduction indrive voltage.

Carrier-transport materials can be selected from known substances aswell as from the carrier-transport materials given above. Note that asthe first to fourth organic compounds, substances having a triplet level(a difference in energy between a ground state and a triplet excitedstate) higher than that of the phosphorescent compound are preferablyselected. An exciplex to be formed exhibits light emission originatingfrom a difference in energy between the shallower HOMO level of the HOMOlevels of the two compounds to be combined and the deeper LUMO level ofthe LUMO levels of the two compounds to be combined; thus, thecombination of the first organic compound and the second organiccompound with which light emission with a desired wavelength can beachieved is selected. In addition, it is preferable that the combinationof the third organic compound and the fourth organic compound beselected so that an exciplex which exhibits light emission whosewavelength overlaps with a wavelength of a lowest-energy-side absorptionband of the phosphorescent compound is formed.

Further, the combination of a material having an electron-transportproperty as one organic compound and a material having a hole-transportproperty as the other organic compound is advantageous for the formationof an exciplex. The transport property of the light-emitting layer canbe easily adjusted and a recombination region can be easily adjusted bychanging the contained amount of each compound. The ratio of thecontained amount of the material having an electron-transport propertyto the contained amount of the material having an electron-transportproperty may be 1:9 to 9:1.

The light-emitting layer 113 having the above-described structure can beformed by co-evaporation by a vacuum evaporation method, or an inkjetmethod, a spin coating method, a dip coating method, or the like using amixed solution.

Note that although the structure in which the first light-emitting layer113 a is formed on the anode side and the second light-emitting layer113 b is formed on the cathode side is described in this embodiment, thestacking order may be reversed. In other words, the secondlight-emitting layer 113 b may be formed on the anode side and the firstlight-emitting layer 113 a may be formed on the cathode side.

Further, the second light-emitting layer 113 b may be fon red of twolayers which have different ratios of the contained amount of the thirdorganic compound to the contained amount of the fourth organic compound.This enables luminance degradation of the light-emitting element to befurther controlled.

The other structure and effect of the light-emitting layer 113 are thesame as those described in Embodiment 1. Embodiment 1 is to be referredto.

The electron-transport layer 114 is a layer containing a substancehaving an electron-transport property. Example of the electron-transportlayer 114 is a layer containing a metal complex having a quinolineskeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo [h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Other than the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis [5-(p-tert-butylphenyl)-1,3 ,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can be used. Thesubstances given here have high electron-transport properties and aremainly ones having an electron mobility of 10⁻⁶ cm²/Vs or more. Notethat any of the above-described host materials having electron-transportproperties may be used for the electron-transport layer 114.

The electron-transport layer 114 is not limited to a single layer andmay be a stack of two or more layers containing any of the substancesgiven above.

Further, a layer for controlling transport of electron carriers may beprovided between the electron-transport layer and the light-emittinglayer. This is a layer formed by addition of a small amount of asubstance having a high electron-trapping property to the aforementionedmaterials having a high electron-transport property, and the layer iscapable of adjusting carrier balance by retarding transport of electroncarriers. Such a structure is very effective in preventing a problem(such as a reduction in element lifetime) caused when electrons passthrough the light-emitting layer.

In addition, an electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an ink-jet method, a spin coating method orthe like may be used. A different formation method may be employed foreach electrode or each layer.

In addition, the electrode may be fonned by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference between the first electrode101 and the second electrode 102, and holes and electrons recombine inthe light-emitting layer 113 which contains a substance with a highlight-emitting property, so that light is emitted. That is, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113, are formed usinga substance having a wider band gap than the light-emitting substance ofthe light-emitting layer or the emission center substance included inthe light-emitting layer.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 102 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Next, an embodiment of a light-emitting element with a structure inwhich a plurality of light-emitting units are stacked (hereinafter thistype of light-emitting element is also referred to as a stacked element)is described with reference to FIG. 1B. In this light-emitting element,a plurality of light-emitting units are provided between a firstelectrode and a second electrode. One light-emitting unit has astructure similar to that of the EL layer 103, which is illustrated inFIG. 1A. In other words, the light-emitting element illustrated in FIG.1A includes a single light-emitting unit; the light-emitting element inthis embodiment includes a plurality of light-emitting units.

In FIG. 1B, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the first electrode 101 and the second electrode 102illustrated in FIG. 1A, and the materials given in the description forFIG. 1A can be used. Further, the first light-emitting unit 511 and thesecond light-emitting unit 512 may have the same structure or differentstructures.

The charge generation layer 513 includes a composite material of anorganic compound and a metal oxide. As the composite material of theorganic compound and the metal oxide, the composite material which canbe used for the hole-injection layer 111 illustrated in FIG. 1A can beused. As the organic compound, a variety of compounds such as anaromatic amine compound, a carbazole compound, aromatic hydrocarbon, anda high molecular compound (an oligomer, a dendrimer, a polymer, or thelike) can be used. A compound having a hole mobility of 1×10⁻⁶ cm²/Vs ormore is preferably used as the organic compound having a hole-transportproperty. Note that any other substance may be used as long as thesubstance has a hole-transport property higher than anelectron-transport property. The composite material of the organiccompound and the metal oxide can achieve low-voltage driving andlow-current driving because of the superior carrier-injecting propertyand carrier-transporting property. Note that when a surface of alight-emitting unit on the anode side is in contact with a chargegeneration layer, the charge generation layer can also serve as ahole-transport layer of the light-emitting unit; thus, a hole-transportlayer does not need to be formed in the light-emitting unit.

The charge generation layer 513 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing another material. For example, alayer containing a composite material of the organic compound and themetal oxide may be combined with a layer containing a compound of asubstance selected from substances with an electron-donating propertyand a compound with a high electron-transport property. Moreover, alayer containing a composite material of the organic compound and themetal oxide may be combined with a transparent conductive film.

The charge generation layer 513 interposed between the firstlight-emitting unit 511 and the second light-emitting unit 512 may haveany structure as long as electrons can be injected to a light-emittingunit on one side and holes can be injected to a light-emitting unit onthe other side when a voltage is applied between the first electrode 501and the second electrode 502. For example, in FIG. 1B, any layer can beused as the charge generation layer 513 as long as the layer injectselectrons into the first light-emitting unit 511 and holes into thesecond light-emitting unit 512 when a voltage is applied such that thepotential of the first electrode is higher than that of the secondelectrode.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 1B; however, the present invention can besimilarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge generation layer between a pair ofelectrodes as in the light-emitting element according to thisembodiment, it is possible to provide a light-emitting element which canemit light with high luminance with the current density kept low and hasa long lifetime. Moreover, a light-emitting device having low powerconsumption, which can be driven at low voltage, can be achieved.

The light-emitting units emit light having different colors from eachother, thereby obtaining light emission of a desired color in the wholelight-emitting element. For example, in a light-emitting element havingtwo light-emitting units, the emission colors of the firstlight-emitting unit are red and green and the emission color of thesecond light-emitting unit is blue, so that the light-emitting elementcan emit white light as the whole element.

When the above-described structure of the light-emitting layer 113 isapplied to at least one of the plurality of units, the number ofmanufacturing steps of the unit can be reduced; thus, a multicolorlight-emitting element which is advantageous for practical applicationcan be provided.

The above-described structure can be combined with any of the structuresin this embodiment and the other embodiments.

Embodiment 3

In this embodiment, a light-emitting device including the light-emittingelement described in Embodiment 1 or 2 is described.

In this embodiment, the light-emitting device manufactured using thelight-emitting element described in Embodiment 1 or 2 is described withreference to FIGS. 2A and 2B. Note that FIG. 2A is a top viewillustrating the light-emitting device and FIG. 2B is a cross-sectionalview of FIG. 2A taken along lines A-B and C-D. This light-emittingdevice includes a driver circuit portion (source line driver circuit)601, a pixel portion 602, and a driver circuit portion (gate line drivercircuit) 603, which are to control light emission of the light-emittingelement and illustrated with dotted lines. Moreover, a reference numeral604 denotes a sealing substrate; 605, a sealing material; and 607, aspace surrounded by the sealing material 605.

Note that a lead wiring 608 is a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and for receiving a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.2B. The driver circuit portion and the pixel portion are formed over anelement substrate 610; the source line driver circuit 601, which is adriver circuit portion, and one of the pixels in the pixel portion 602are illustrated here.

In the source line driver circuit 601, a CMOS circuit is formed in whichan n-channel TFT 623 and a p-channel TFT 624 are combined. In addition,the driver circuit may be formed with any of a variety of circuits suchas a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although adriver-integrated type in which the driver circuit is formed over thesubstrate is described in this embodiment, the present invention is notlimited to this type and the driver circuit can be formed outside thesubstrate.

The pixel portion 602 is formed with a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 connected electrically with a drain of the current controlling TFT.An insulator 614 is formed to cover the end portions of the firstelectrode 613. Here, the insulator 614 is formed using a positive typephotosensitive acrylic resin film

In order to improve the coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. As a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

In addition, the EL layer 616 is formed by any of a variety of methodssuch as an evaporation method using an evaporation mask, an inkjetmethod, and a spin coating method. The EL layer 616 has the structuredescribed in Embodiment 1 or 2. Further, for another material includedin the EL layer 616, any of low molecular-weight compounds and polymericcompounds (including oligomers and dendrimers) may be used.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, Mgln, or AlLi) is preferably used. In the case where lightgenerated in the EL layer 616 is transmitted through the secondelectrode 617, a stack of a thin metal film and a transparent conductivefilm (e.g., ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt%, indium tin oxide containing silicon, or zinc oxide (ZnO)) ispreferably used for the second electrode 617.

Note that the light-emitting element is foamed with the first electrode613, the EL layer 616, and the second electrode 617. The light-emittingelement has the structure described in Embodiment 1 or 2. In thelight-emitting device of this embodiment, the pixel portion, whichincludes a plurality of light-emitting elements, may include both thelight-emitting element described in Embodiment 1 or 2 and alight-emitting element having a different structure.

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that the light-emitting element618 is provided in the space 607 surrounded by the element substrate610, the sealing substrate 604, and the sealing material 605. The space607 may be filled with filler, or may be filled with an inert gas (suchas nitrogen or argon), or the sealing material 605. It is preferablethat the sealing substrate be provided with a recessed portion and thedesiccant 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiberglass reinforced plastic (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in Embodiment 1 or 2 can be obtained.The light-emitting device in this embodiment is fabricated using thelight-emitting element described in Embodiment 1 or 2 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 or 2 has favorable emissionefficiency, the light-emitting device can have reduced powerconsumption. In addition, since the light-emitting element is easy tomass-produce, the light-emitting device can be provided at low cost.

FIGS. 3A and 3B each illustrate an example of a light-emitting device inwhich full color display is achieved by formation of a light-emittingelement exhibiting white light emission and with the use of coloringlayers (color filters) and the like. In FIG. 3A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G and 1024B of light-emitting elements, a partition 1025, anEL layer 1028, a second electrode 1029 of the light-emitting elements, asealing substrate 1031, a sealing material 1032, and the like areillustrated.

In FIG. 3A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black layer (a black matrix) 1035 maybe additionally provided. The transparent base material 1033 providedwith the coloring layers and the black layer is positioned and fixed tothe substrate 1001. Note that the coloring layers and the black layerare covered with an overcoat layer 1036. In FIG. 3A, light emitted frompart of the light-emitting layer does not pass through the coloringlayers, while light emitted from the other part of the light-emittinglayer passes through the coloring layers. Since light which does notpass through the coloring layers is white and light which passes throughany one of the coloring layers is red, blue, or green, an image can bedisplayed using pixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As in the structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are fanned (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 4is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming of a connection electrode which connects the TFT and the anodeof the light-emitting element is performed in a manner similar to thatof the light-emitting device having a bottom emission structure. Then athird interlayer insulating film 1037 is formed to cover an electrode1022. This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, and can alternativelybe fonned using any other known material.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each function as an anode here, but may functionas a cathode. Further, in the case of a light-emitting device having atop emission structure as illustrated in FIG. 4, the first electrodesare preferably reflective electrodes. The EL layer 1028 is formed tohave a structure similar to the structure of the EL layer 103, which isdescribed in Embodiment 1 or 2, with which white light emission can beobtained.

In the case of a top emission structure as illustrated in FIG. 4,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black layer (the black matrix)1035 which is positioned between pixels. The coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G and the bluecoloring layer 1034B) and the black layer (the black matrix) may becovered with the overcoat layer 1036. Note that a light-transmittingsubstrate is used as the sealing substrate 1031.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

The light-emitting device in this embodiment is manufactured using thelight-emitting element described in Embodiment 1 or 2 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 or 2 has favorable emissionefficiency, the light-emitting device can have reduced powerconsumption. In addition, since the light-emitting element is easy tomass-produce, the light-emitting device can be provided at low cost.

An active matrix light-emitting device is described above, whereas apassive matrix light-emitting device is described below. FIGS. 5A and 5Billustrate a passive matrix light-emitting device manufactured using thepresent invention. FIG. 5A is a perspective view of the light-emittingdevice, and FIG. 5B is a cross-sectional view of FIG. 5A taken alongline X-Y. In FIGS. 5A and 5B, an EL layer 955 is provided between anelectrode 952 and an electrode 956 over a substrate 951. An end portionof the electrode 952 is covered with an insulating layer 953. Apartition layer 954 is provided over the insulating layer 953. Thesidewalls of the partition layer 954 are aslope such that the distancebetween both sidewalls is gradually narrowed toward the surface of thesubstrate. In other words, a cross section taken along the direction ofthe short side of the partition wall layer 954 is trapezoidal, and thelower side (a side which is in the same direction as a plane directionof the insulating layer 953 and in contact with the insulating layer953) is shorter than the upper side (a side which is in the samedirection as the plane direction of the insulating layer 953 and not incontact with the insulating layer 953. The partition layer 954 thusprovided can prevent defects in the light-emitting element due to staticelectricity or the like. Further, also in the passive matrixlight-emitting device, the light-emitting element described inEmbodiment 1 or 2, which has favorable emission efficiency, is used, sothat the light-emitting device can have less power consumption.Moreover, since the light-emitting element is easy to mass-produce, thelight-emitting device can be provided at low cost.

Since many minute light-emitting elements arranged in a matrix in thelight-emitting device described above can each be controlled, thelight-emitting device can be suitably used as a display device fordisplaying images.

This embodiment can be freely combined with any of other embodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a lighting device isdescribed with reference to FIGS. 6A and 6B. FIG. 6B is a top view ofthe lighting device, and FIG. 6A is a cross-sectional view of FIG. 6Ataken along line e-f.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 1. When light is extracted through thefirst electrode 401 side, the first electrode 401 is formed using amaterial having a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

An EL layer 403 is formed over the first electrode 401. The structure ofthe EL layer 403 corresponds to, for example, the structure of the ELlayer 103 in Embodiment 1, or the structure in which the light-emittingunits 511 and 512 and the charge-generation layer 513 are combined. Forthese structures, the description in Embodiment 1 can be referred to.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in Embodiment 1.The second electrode 404 is formed using a material having highreflectance when light is extracted through the first electrode 401side. The second electrode 404 is connected to the pad 412, wherebyvoltage is applied thereto.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404. Since the light-emittingelement has high emission efficiency, the lighting device in thisembodiment can be a lighting device having low power consumption.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealing material 405 or the sealing material 406. Inaddition, the inner sealing material 406 (not shown in FIG. 6B) can bemixed with a desiccant which enables moisture to be adsorbed, increasingreliability.

When parts of the pad 412 and the first electrode 401 are extended tothe outside of the sealing materials 405 and 406, the extended parts canserve as external input terminals. An IC chip 420 mounted with aconverter or the like may be provided over the external input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in Embodiment 1or 2 as an EL element, the lighting device can be a lighting devicehaving low power consumption. Further, the lighting device can be alighting device driven at low voltage. The lighting device can also beinexpensive.

Embodiment 5

In this embodiment, examples of electronic appliances each including thelight-emitting element described in Embodiment 1 or 2 are described. Thelight-emitting element described in Embodiment 1 or 2 has favorableemission efficiency and reduced power consumption. As a result, theelectronic appliances described in this embodiment can each include alight-emitting portion having reduced power consumption. Thelight-emitting element described in Embodiment 1 or 2 includes a smallernumber of layers to be formed; thus, the electronic appliance can beinexpensive.

Examples of the electronic appliance to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic appliances are described below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting elements described in Embodiment 1 or 2 are arranged in amatrix. The light-emitting elements can have favorable emissionefficiency. Further, the light-emitting elements can be driven at lowvoltage. Moreover, the light-emitting elements can have a long lifetime.Therefore, the television device including the display portion 7103which is formed using the light-emitting element can have reduced powerconsumption. Further, the television device can be driven at lowvoltage. Moreover, the television device can have high reliability.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Further, the remote controller 7110 may be provided with adisplay portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 7B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using light-emitting elements arranged ina matrix in the display portion 7203, which are the same as thatdescribed in Embodiment 1 or 2. The computer illustrated in FIG. 7B1 mayhave a structure illustrated in FIG. 7B2. The computer illustrated inFIG. 7B2 is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 is a touch screen, and input can be performed by operation ofdisplay for input on the second display portion 7210 with a finger or adedicated pen. The second display portion 7210 can also display imagesother than the display for input. The display portion 7203 may also be atouchscreen. Connecting the two screens with a hinge can preventtroubles; for example, the screens can be prevented from being crackedor broken while the computer is being stored or carried. Note that thiscomputer is manufactured using light-emitting elements arranged in amatrix in the display portion 7203, which are the same as that describedin Embodiment 2 or 3. The light-emitting elements can have favorableemission efficiency. Therefore, this computer having the display portion7203 which is formed using the light-emitting elements consumes lesspower.

FIG. 7C illustrates a portable game machine, which includes twohousings, a housing 7301 and a housing 7302, which are connected with ajoint portion 7303 so that the portable game machine can be opened orfolded. The housing 7301 incorporates a display portion 7304 includingthe light-emitting elements each of which is described in Embodiment 1or 2 and which are arranged in a matrix, and the housing 7302incorporates a display portion 7305. In addition, the portable gamemachine illustrated in FIG. 7C includes a speaker portion 7306, arecording medium insertion portion 7307, an LED lamp 7308, an inputmeans (an operation key 7309, a connection terminal 7310, a sensor 7311(a sensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), anda microphone 7312), and the like. Needless to say, the structure of theportable game machine is not limited to the above as long as the displayportion including the light-emitting elements each of which is describedin Embodiment 1 or 2 and which are arranged in a matrix is used as atleast either the display portion 7304 or the display portion 7305, orboth, and the structure can include other accessories as appropriate.The portable game machine illustrated in FIG. 7C has a function ofreading out a program or data stored in a storage medium to display iton the display portion, and a function of sharing information withanother portable game machine by wireless communication. The portablegame machine illustrated in FIG. 7C can have a variety of functionswithout limitation to the above. The portable game machine having thedisplay portion 7304 can consume less power because the light-emittingelements used in the display portion 7304 have favorable emissionefficiency. Since the light-emitting elements used in the displayportion 7304 has low driving voltage, the portable game machine can alsobe a portable game machine having low driving voltage. Furthermore,since the light-emitting elements used in the display portion 7304 hashigh reliability, the portable game machine can also have highreliability.

FIG. 7D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400has the display portion 7402 including the light-emitting elements eachof which is described in Embodiment 1 or 2 and which are arranged in amatrix. The light-emitting elements can have favorable emissionefficiency. In addition, the light-emitting element can have low drivingvoltage. Furthermore, the light-emitting element can have a longlifetime. Therefore, this mobile phone having the display portion 7402which is formed using the light-emitting elements consumes less power.In addition, the mobile phone can have low driving voltage. Furthermore,the mobile phone can have high reliability.

When the display portion 7402 of the mobile phone illustrated in FIG. 7Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touch on the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device which includes a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, is providedinside the mobile phone, the direction of the cellular phone (whetherthe cellular phone is placed horizontally or vertically for a landscapemode or a portrait mode) is determined so that display on the screen ofthe display portion 7402 can be automatically switched.

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in Embodiment 1 or 2 is wideso that this light-emitting device can be applied to electronicappliances in a variety of fields. By using the light-emitting elementdescribed in Embodiment 1 or 2, an electronic appliance having reducedpower consumption can be obtained.

FIG. 8 illustrates an example of a liquid crystal display device usingthe light-emitting element described in Embodiment 1 or 2 for abacklight. The liquid crystal display device shown in FIG. 8 includes ahousing 901, a liquid crystal layer 902, a backlight unit 903, and ahousing 904. The liquid crystal layer 902 is connected to a driver IC905. The light-emitting element described in Embodiment 1 or 2 is usedfor the backlight unit 903, to which current is supplied through aterminal 906.

The light-emitting element described in Embodiment 1 or 2 is used forthe backlight of the liquid crystal display device; thus, the backlightcan have reduced power consumption. In addition, the use of thelight-emitting element described in Embodiment 2 enables manufacture ofa planar-emission lighting device and further a larger-areaplanar-emission lighting device; therefore, the backlight can be alarger-area backlight, and the liquid crystal display device can also bea larger-area device. Furthermore, the light-emitting device using thelight-emitting element described in Embodiment 2 can be thinner than aconventional one; accordingly, the display device can also be thinner.

FIG. 9 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a table lamp which is alighting device. The table lamp illustrated in FIG. 9 includes a housing2001 and a light source 2002, and the lighting device described inEmbodiment 4 is used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for an indoor lighting device3001. Since the light-emitting element described in Embodiment 1 or 2has reduced power consumption, a lighting device having reduced powerconsumption can be obtained. Further, since the light-emitting elementdescribed in Embodiment 1 or 2 can have a large area, the light-emittingelement can be used for a large-area lighting device. Furthermore, sincethe light-emitting element described in Embodiment 1 or 2 is thin, thelight-emitting element can be used for a lighting device having areduced thickness.

The light-emitting element described in Embodiment 1 or 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 11illustrates one mode in which the light-emitting element described inEmbodiment 2 is used for an automobile windshield and an automobiledashboard. Display regions 5000 to 5005 each include the light-emittingelements described in Embodiments 1 or 2.

The display regions 5000 and the display region 5001 are provided in theautomobile windshield in which the light-emitting elements described inEmbodiment 1 or 2 are incorporated. The light-emitting element describedin Embodiment 1 or 2 can be formed into what is called a see-throughdisplay device, through which the opposite side can be seen, byincluding a first electrode and a second electrode formed of electrodeshaving light-transmitting properties. Such see-through display devicescan be provided even in the automobile windshield, without hindering thevision. Note that in the case where a transistor for driving or the likeis provided, a transistor having a light-transmitting property, such asan organic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display device incorporating the light-emitting element described inEmbodiment 1 or 2 is provided in the display region 5002 in a pillarportion. The display region 5002 can compensate for the view hindered bythe pillar portion by showing an image taken by an imaging unit providedin the car body. Similarly, the display region 5003 provided in thedashboard can compensate for the view hindered by the car body byshowing an image taken by an imaging unit provided in the outside of thecar body, which leads to elimination of blind areas and enhancement ofsafety. Showing an image so as to compensate for the area which a drivercannot see makes it possible for the driver to confirm safety easily andcomfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation data, a speedometer,a tachometer, a mileage, a fuel meter, a gearshift indicator, andair-condition setting. The content or layout of the display can bechanged freely by a user as appropriate. Further, such information canalso be shown by the display regions 5000 to 5003. Note that the displayregions 5000 to 5005 can also be used as lighting devices.

The light-emitting element described in Embodiment 1 or 2 can have highemission efficiency and low power consumption. Therefore, load on abattery is small even when a number of large screens such as the displayregions 5000 to 5005 are provided, which provides comfortable use. Forthat reason, the light-emitting device and the lighting device each ofwhich includes the light-emitting element described in Embodiment 1 or 2can be suitably used as an in-vehicle light-emitting device and anin-vehicle lighting device.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal.The tablet terminal is opened in FIG. 12A. The tablet terminal includesa housing 9630, a display portion 9631 a, a display portion 9631 b, adisplay mode switch 9034, a power switch 9035, a power saver switch9036, a clasp 9033, and an operation switch 9038. Note that in thetablet terminal, one or both of the display portion 9631 a and thedisplay portion 9631 b is/are formed using a light-emitting device whichincludes the light-emitting element described in Embodiment 1 or 2.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. A switching button 9639 forshowing/hiding a keyboard of the touch panel is touched with a finger, astylus, or the like, so that keyboard buttons can be displayed on thedisplay portion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. With the switch 9036 for switching topower-saving mode, the luminance of display can be optimized inaccordance with the amount of external light at the time when the tabletis in use, which is detected with an optical sensor incorporated in thetablet. The tablet may include another detection device such as a sensorfor detecting orientation (e.g., a gyroscope or an acceleration sensor)in addition to the optical sensor.

Although FIG. 12A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, one of themmay be a display panel that can display higher-definition images thanthe other.

The tablet terminal is folded in FIG. 12B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DC-to-DC converter 9636. Note thatFIG. 12B illustrates an example in which the charge and dischargecontrol circuit 9634 includes the battery 9635 and the DC-to-DCconverter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet terminal with high endurance andhigh reliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 12A and 12B canhave a function of displaying various kinds of information (e.g., astill image, a moving image, and a text image) on the display portion, afunction of displaying a calendar, the date, the time, or the like onthe display portion, a touch input function of operating or editinginformation displayed on the display portion by touch input, a functionof controlling processing by various kinds of software (programs), andthe like.

The solar cell 9633, which is attached on the surface of the tabletteriiiinal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633is preferably provided on one or two surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 12B are described with reference to a blockdiagram of FIG. 12C. FIG. 12C shows the solar cell 9633, the battery9635, the DC-to-DC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 12B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DC-to-DCconverter 9636 so that the power has voltage for charging the battery9635. Then, when power supplied from the battery 9635 charged by thesolar cell 9633 is used for the operation of the display portion 9631,the switch SW1 is turned on and the voltage of the power is raised orlowered by the converter 9638 so as to be voltage needed for the displayportion 9631. In addition, when display on the display portion 9631 isnot performed, the switch SW1 is turned off and a switch SW2 is turnedon so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge means used in combination, and the power generation meansis not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 12A to 12C as long as thedisplay portion 9631 is included.

EXAMPLE 1

In this example, light-emitting elements (light-emitting elements 1 to3) each of which is one embodiment of the present invention aredescribed. Chemical formulae of materials used in this example are shownbelow. Note that each of the light-emitting elements 1 to 3 includes alight-emitting layer formed of two light-emitting layers (a firstlight-emitting layer and a second light-emitting layer) which are incontact with each other. The first light-emitting layer has thestructure described in Embodiment 1, in which light emission is obtainedfrom an exciplex, and the second light-emitting layer has the structuredescribed in Embodiment 1, in which light emission is obtained from aphosphorescent compound.

Methods for manufacturing the light-emitting elements 1 to 3 of thisexample are described below.

(Method for Manufacturing Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method to form the firstelectrode 101. The thickness of the first electrode 101 was set to 110run and the electrode area was set to 2 mm×2 mm Here, the firstelectrode 101 functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water, baked at200° C. for one hour, and then subjected to UV ozone treatment for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then cooled down forabout 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknessof the hole-injection layer 111 was set to 20 nm, and the weight ratioof DBT3P-II to molybdenum oxide was adjusted to 4:2 (=DBT3P-II:molybdenum oxide). Note that the co-evaporation method refers to anevaporation method in which evaporation is carried out from a pluralityof evaporation sources at the same time in one treatment chamber.

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) represented by Structural Formula (ii) was deposited to athickness of 20 nm on the hole-injection layer 111 to form thehole-transport layer 112.

Furthermore, on the hole-transport layer 112,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f;h]quinoxaline (abbreviation:2mDBTPDBq-II) represented by Structural Formula (iii) andN,N′-bis(9-phenyl-9H-carbazol-3-yl)-N,N′-diphenyl-spiro-9,9′-bifluorene-2,7-diamine(abbreviation: PCA2SF) represented by Structural Formula (iv) weredeposited to a thickness of 20 nm by co-evaporation so that the weightratio of 2mDBTPDBq-II to PCA2SF was 0.8:0.2 (=2mDBTPDBq-II: PCA2SF) toform the first light-emitting layer 113 a, and then 2mDBTPDBq-II,PCA2SF, and bis(2,3 ,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)]) represented by Structural Foimula(v) were deposited to a thickness of 20 nm by co-evaporation so that theweight ratio of 2mDBTPDBq-II to PCA2SF and [Ir(tppr)₂(dpm)] was0.8:0.2:0.025 (=2mDBTPDBq-II: PCA2SF: [Ir(tppr)₂(dpm)]) to form thesecond light-emitting layer 113 b. In the above-described manner, thelight-emitting layer 113 was formed. Note that 2mDBTPDBq-II and PCA2SFform an exciplex.

After that, on the light-emitting layer 113, 2mDBTPDBq-II was depositedto a thickness of 20 nm, and bathophenanthroline (abbreviation: BPhen)represented by Structural Formula (v) was deposited to a thickness of 20nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 mn toform the electron-injection layer 115. Finally, aluminum was depositedby evaporation to a thickness of 200 nm to form the second electrode 102functioning as a cathode. Through the above-described steps, thelight-emitting element 1 of this example was manufactured.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method for Manufacturing Light-Emitting Element 2)

The light-emitting element 2 was manufactured in such a manner that2mDBTPDBq-II and [Ir(tppr)₂(dpm)] were deposited to a thickness of 20 nmby co-evaporation so that the weight ratio of 2mDBTPDBq-II to[Ir(tppr)₂(dpm)] was 1:0.025 (=2mDBTPDBq-II: [Ir(tppr)₂(dpm)]) to formthe second light-emitting layer 113 b. The structures and manufacturingmethods of the other components were the same as those of thelight-emitting element 1.

(Method for Manufacturing Light-Emitting Element 3)

The light-emitting layer 113 in the light-emitting element 3 was formedin such a manner that 2mDBTPDBq-II and2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF) represented by Structural Forinula (vii) weredeposited by co-evaporation to a thickness of 20 mn so that the weightratio of 2mDBTPDBq-II to PCASF was 0.8:0.2 (=2mDBTPDBq-II: PCASF) tofaun the first light-emitting layer 113 a, and then 2mDBTPDBq-II, PCASF,and [Ir(tppr)₂(dpm)] were deposited by co-evaporation to a thickness of20 nm so that the weight ratio of 2mDBTPDBq-II to PCASF and[Ir(tppr)₂(dpm)] was 0.8:0.2:0.025 (=2mDBTPDBq-II: PCASF:[Ir(tppr)₂(dpm)]) to form the second light-emitting layer 113 b. Inother words, the light-emitting element 3 was manufactured using PCASFinstead of PCA2SF used in the light-emitting element 1. Note that thestructures and manufacturing methods of the other components are thesame as those of the light-emitting element 1. Note that 2mDBTPDBq-IIand PCASF form an exciplex.

In a glove box under a nitrogen atmosphere, the light-emitting elements1 to 3 were each sealed with a glass substrate so as not to be exposedto the air (specifically, a sealing material was applied onto an outeredge of the element, UV treatment was performed at the time of sealing,and heat treatment was performed at 80° C. for one hour). Then, thereliability of each of the light-emitting elements was measured. Notethat the measurement was carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 13, FIG. 14, FIG. 15, FIG. 16, and FIG. 17 show theluminance-current density characteristics of the light-emitting element1, the current efficiency-luminance characteristics thereof, theluminance-voltage characteristics thereof, the external quantumefficiency-luminance characteristics thereof, and the emission spectrumthereof, respectively.

FIG. 18, FIG. 19, FIG. 20, FIG. 21, and FIG. 22 show theluminance-current density characteristics of the light-emitting element2, the current efficiency-luminance characteristics thereof, theluminance-voltage characteristics thereof, the external quantumefficiency-luminance characteristics thereof, and the emission spectrumthereof, respectively.

FIG. 23, FIG. 24, FIG. 25, FIG. 26, and FIG. 27 show theluminance-current density characteristics of the light-emitting element3, the current efficiency-luminance characteristics thereof, theluminance-voltage characteristics thereof, the external quantumefficiency-luminance characteristics thereof, and the emission spectrumthereof, respectively.

According to the characteristics, the light-emitting elements 1 to 3each have a current efficiency of 20 cd/A or higher and favorableemission efficiency with an external quantum efficiency of 10% or higherat around 1000 cd/m².

Further, in the emission spectra, red light emission originating from[Ir(tppr)₂(dpm)] and green light emission (a shoulder at around 550 nm)originating from an exciplex are observed. This indicates that lightemission is obtained from both the first light-emitting layer 113 a andthe second light-emitting layer 113 b.

EXAMPLE 2

In this example, a light-emitting element (a light-emitting element 4)of one embodiment of the present invention is described. Chemicalformulae of materials used in this example are shown below. Note thatthe light-emitting element 4 includes a light-emitting layer in whichtwo light-emitting layers (a first light-emitting layer and a secondlight-emitting layer) are formed in contact with each other. The firstlight-emitting layer has the structure described in Embodiment 1, inwhich light emission is obtained from a fluorescent compound to whichenergy has been transferred from an exciplex, and the secondlight-emitting layer has the structure described in Embodiment 1, inwhich light emission is obtained from a phosphorescent compound.

A method for manufacturing the light-emitting element 4 of this exampleis described below.

(Method for Manufacturing Light-emitting Element 4)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasfomied over a glass substrate by a sputtering method to form the firstelectrode 101. The thickness of the first electrode 101 was set to 110nm and the electrode area was set to 2 mm×2 mm. Here, the firstelectrode 101 functions as an anode of the light-emitting

Next, as pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water, baked at200° C. for one hour, and then subjected to UV ozone treatment for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then cooled down forabout 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknessof the hole-injection layer 111 was set to 40 nm, and the weight ratioof DBT3P-II to molybdenum oxide was adjusted to 4:2 (=DBT3P-II:molybdenum oxide). Note that the co-evaporation method refers to anevaporation method in which evaporation is carried out from a pluralityof evaporation sources at the same time in one treatment chamber.

Next, 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBNBB) represented by Structural Formula (viii) wasdeposited to a thickness of 20 nm on the hole-injection layer 111 toform the hole-transport layer 112.

Furthermore, on the hole-transport layer 112,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo [f,h]quinoxaline(abbreviation: 2mDBTBPDB q-II) represented by Structural Formula (ix),PCBNBB, andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(tBuppm)₂(acac)]) represented by StructuralFormula (x) were deposited by co-evaporation to a thickness of 20 nm sothat the weight ratio of 2mDBTBPDBq-II to PCBNBB and [Ir(tBuppm)₂(acac)]was 0.8:0.2:0.05 (=2mDBTBPDBq-II: PCBNBB: [Ir(tBuppm)₂(acac)]) and then2mDBTBPDBq-II, PCBNBB, and [Ir(tppr)₂(dpm)] were deposited byco-evaporation to a thickness of 5 nm so that the weight ratio of2mDBTBPDBq-II to PCBNBB and [Ir(tppr)₂(dpm)] was 0.9:0.1:0.05(=2mDBTBPDBq-II: PCBNBB: [Ir(tBuppm)₂(acac)]) to form the secondlight-emitting layer 113 b, and 2mDBTBPDBq-II, PCBNBB, andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula(xi) were deposited by co-evaporation to a thickness of 25 mn so thatthe weight ratio of 2mDBTBPDBq-II to PCBNBB and 1,6mMemFLPAPrn was0.3:0.7:0.05 to form the first light-emitting layer 113 a. In theabove-described manner, the light-emitting layer 113 was formed. In thelight-emitting element 4, the second light-emitting layer 113 b wasformed on the hole-transport layer 112, and the first light-emittinglayer 113 a was formed on the second light-emitting layer 113 b.

Note that 2mDB IBPDBq-II and PCBNBB form an exciplex. FIG. 34 shows anemission spectrum of a film of 2mDBTBPDBq-II alone, an emission spectrumof a film of PCBNBB alone, and an emission spectrum of a film formed byco-evaporation of 2mDBTBPDBq-II and PCBNBB. As shown in FIG. 34, theposition and the shape of the emission spectrum of the film formed byco-evaporation of 2mDBTBPDBq-II and PCBNBB are different from those ofthe emission spectra of the film of 2mDBTBPDBq-II alone and the film ofPCBNBB alone. Further, the emission spectrum is located in a longerwavelength region than the emission spectra of the film of 2mDBTBPDBq-IIalone and the film of PCBNBB alone. This indicates that an exciplex isformed by 2mDBTBPDBq-II and PCBNBB.

After that, on the light-emitting layer 113, 2mDBTBPDBq-II was depositedto a thickness of 15 nm, and bathophenanthroline (abbreviation: BPhen)represented by Structural Formula (v) was deposited to a thickness of 15nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm toform the electron-injection layer 115. Finally, aluminum was depositedby evaporation to a thickness of 200 nm to form the second electrode 102functioning as a cathode. Through the above-described steps, thelight-emitting element 4 of this example was manufactured.

Note that in all the above evaporation steps, evaporation was perfonnedby a resistance-heating method.

In a glove box under a nitrogen atmosphere, the light-emitting element 4was sealed with a glass substrate so as not to be exposed to the air(specifically, a sealing material was applied onto an outer edge of theelement, UV treatment was performed at the time of sealing, and heattreatment was performed at 80° C. for one hour). Then, the reliabilityof the light-emitting element was measured. Note that the measurementwas carried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 28, FIG. 29, FIG. 30, FIG. 31, and FIG. 32 show theluminance-current density characteristics of the light-emitting element4, the current efficiency-luminance characteristics thereof, theluminance-voltage characteristics thereof, the external quantumefficiency-luminance characteristics thereof, and an emission spectrumthereof, respectively.

According to the characteristics, the light-emitting element 4 hasfavorable emission efficiency with a current efficiency of 20 cd/A orhigher and an external quantum efficiency of 10% or higher at around1000 cd/m².

Further, according to the emission spectrum, red light emissionoriginating from [Ir(tppr)₂(dpm)], green light emission originating from[Ir(tBuppm)₂(acac)], and blue light emission originating from1,6mMemFLPAPm are observed. This indicates that light emission isobtained from both of the first light-emitting layer 113 a and thesecond light-emitting layer 113 b and that the light-emitting element inwhich the fluorescent substance is used as an emission center substancein the first light-emitting layer 113 a is able to obtain favorablecharacteristics.

FIG. 33 shows the result of a reliability test in which thelight-emitting element 4 was driven under conditions that the initialluminance was 3000 cd/m² and the current density was constant. FIG. 33shows a change in normalized luminance from an initial luminance of100%. The result shows that the light-emitting element 4 kept 86% of theinitial luminance even after being driven for 350 hours, which meansthat the light-emitting element 4 has small luminance degradation withdriving time and excellent reliability.

EXPLANATION OF REFERENCE

10: electde, 11: electrode, 101: first electrode, 102: second electrode,103: EL layer, 111: hole-injection layer, 112: hole-transport layer,113: light-emitting layer, 113 a: first light-emitting layer, 113 b:second light-emitting layer, 114: electron-transport layer, 115:electron-injection layer, 400: substrate, 401: first electrode, 403: ELlayer, 404: second electrode, 405: sealing material, 406: sealingmaterial, 407: sealing substrate, 412: pad, 420: IC chip, 501: firstelectrode, 502: second electrode, 511: first light-emitting unit, 512:second light-emitting unit, 513: charge generation layer, 601: drivercircuit portion (source line driver circuit), 602: pixel portion, 603:driver circuit portion (gate line driver circuit), 604: sealingsubstrate, 605: sealing material, 607: space, 608: wiring, 609: FPC(flexible printed circuit), 610: element substrate, 611: switching TFT,612: current controlling TFT, 613: first electrode, 614: insulator, 616:EL layer, 617: second electrode, 618: light-emitting element, 623:n-channel TFT, 624: p-channel TFT, 625: desiccant, 901: housing, 902:liquid crystal layer, 903: backlight unit, 904: housing, 905: driver IC,906: terminal, 951: substrate, 952: electrode, 953: insulating layer,954: partition layer, 955: EL layer, 956: electrode, 1001: substrate,1002: base insulating film, 1003: gate insulating film, 1006: gateelectrode, 1007: gate electrode, 1008: gate electrode, 1020: firstinterlayer insulating film, 1021: second interlayer insulating film,1022: electrode, 1024W: first electrode of light-emitting element,1024R: first electrode of light-emitting element, 1024G: first electrodeof light-emitting element, 1024B: first electrode of light-emittingelement, 1025: partition, 1028: EL layer, 1029: second electrode oflight-emitting element, 1031: sealing substrate, 1032: sealing material,1033: transparent base material, 1034R: red coloring layer, 1034G: greencoloring layer, 1034B: blue coloring layer, 1035: black layer (blackmatrix), 1036: overcoat layer, 1037: third interlayer insulating film,1040: pixel portion, 1041: driver circuit portion, 1042: peripheralportion, 2001: housing, 2002: light source, 3001: lighting device, 5000:display region, 5001: display region, 5002: display region, 5003:display region, 5004: display region, 5005: display region, 7101:housing, 7103: display portion, 7105: stand, 7107: display portion,7109: operation key, 7110: remote controller, 7201: main body, 7202:housing, 7203: display portion, 7204: keyboard, 7205: externalconnection port, 7206: pointing device, 7210: second display portion,7301: housing, 7302: housing, 7303: joint portion, 7304: displayportion, 7305: display portion, 7306: speaker portion, 7307: recordingmedium insertion portion, 7308: LED lamp, 7309: operation key, 7310:connection terminal, 7311: sensor, 7401: housing, 7402: display portion,7403: operation button, 7404: external connection port, 7405: speaker,7406: microphone, 7400: mobile phone, 9033: hinge, 9034: switch, 9035:power switch, 9036: switch, 9038: operation switch, 9630: housing, 9631:display portion, 9631 a: display portion, 9631 b: display portion, 9632a: touchscreen region, 9632b: touchscreen region, 9633: solar cell,9634: charge and discharge control circuit, 9635: battery, 9636:DC-to-DC converter, 9637: operation key, 9638: converter, and 9639:button.

This application is based on Japanese Patent Application serial no.2012-173027 filed with Japan Patent Office on Aug. 3, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. (canceled)
 2. A light-emitting elementcomprising: a first electrode, a second electrode, and anelectroluminescence layer interposed between the first electrode and thesecond electrode, wherein the electroluminescence layer comprises alight-emitting layer including a first light-emitting layer and a secondlight-emitting layer over the first light-emitting layer, wherein thefirst light-emitting layer contains a first organic compound, a secondorganic compound, and a fluorescent substance, wherein the secondlight-emitting layer contains a third organic compound and aphosphorescent substance, wherein a combination of the first organiccompound and the second organic compound forms a first exciplex, andwherein the first organic compound and the second organic compound aremixed in the first light-emitting layer to form a single light-emittinglayer.
 3. The light-emitting element according to claim 2, wherein thefirst light-emitting layer is in contact with the second light-emittinglayer,
 4. The light-emitting element according to claim 2, wherein alight emission spectrum of the first exciplex has a peak on a shorterwavelength side than a light emission spectrum of the phosphorescentsubstance.
 5. The light-emitting element according to claim 2, wherein adifference in energy between a triplet excited level and a singletexcited level of the first exciplex is greater than or equal to 0 eV andless than or equal to 0.2 eV.
 6. The light-emitting element according toclaim 2, wherein white light emission is exhibited.
 7. A light-emittingmodule comprising: the light-emitting element according to claim 2, anda means controlling the light-emitting element.
 8. A light-emittingdevice comprising: the light-emitting element according to claim 2, anda means controlling the light-emitting element.
 9. An electronicappliance comprising the light-emitting element according to claim 2.10. A light-emitting element comprising: a first electrode, a secondelectrode, and an electroluminescence layer interposed between the firstelectrode and the second electrode, wherein the electroluminescencelayer comprises a light-emitting layer including a first light-emittinglayer and a second light-emitting layer over the first light-emittinglayer, wherein the first light-emitting layer contains a first organiccompound, a second organic compound, and a fluorescent substance,wherein the second light-emitting layer contains a third organiccompound, a fourth organic compound, and a phosphorescent substance,wherein a combination of the first organic compound and the secondorganic compound fours a first exciplex, and wherein a combination ofthe third organic compound and the fourth organic compound forms asecond exciplex.
 11. The light-emitting element according to claim 10,wherein the first light-emitting layer is in contact with the secondlight-emitting layer,
 12. The light-emitting element according to claim10, wherein a light emission spectrum of the first exciplex has a peakon a shorter wavelength side than a light emission spectrum of thephosphorescent substance.
 13. The light-emitting element according toclaim 10, wherein a difference in energy between a triplet excited leveland a singlet excited level of the first exciplex is greater than orequal to 0 eV and less than or equal to 0.2 eV.
 14. The light-emittingelement according to claim 10, wherein a lowest-energy-side absorptionband of the phosphorescent substance overlaps with an emission spectrumof the second exciplex.
 15. The light-emitting element according toclaim 10, wherein a difference in equivalent energy values between apeak wavelength of a lowest-energy-side absorption band of thephosphorescent substance and a peak wavelength of an emission spectrumof the second exciplex is less than or equal to 0.2 eV.
 16. Thelight-emitting element according to claim 10, wherein one of the firstorganic compound and the second organic compound is a material having anelectron-transport property and the other is a material having ahole-transport property, and wherein one of the third organic compoundand the fourth organic compound is a material having anelectron-transport property and the other is a material having ahole-transport property.
 17. The light-emitting element according toclaim 16, wherein one of the first electrode and the second electrodefunctions as an anode and the other functions as a cathode, wherein oneof the first light-emitting layer and the second light-emitting layer,which is positioned on the side where the electrode functioning as theanode is fon ied, contains a large amount of the material having thehole-transport property, and wherein the other of the firstlight-emitting layer and the second light-emitting layer, which ispositioned on the side where the electrode functioning as the cathode isformed, contains a large amount of the material having theelectron-transport property.
 18. The light-emitting element according toclaim 10, wherein a combination of the first organic compound and thesecond organic compound is the same as a combination of the thirdorganic compound and the fourth organic compound.
 19. The light-emittingelement according to claim 10, wherein a light emission spectrum of thefirst exciplex has a peak on a shorter wavelength side than a lightemission spectrum of the second exciplex.
 20. The light-emitting elementaccording to claim 10, wherein white light emission is exhibited.
 21. Alight-emitting module comprising: the light-emitting element accordingto claim 10, and a means controlling the light-emitting element.
 22. Alight-emitting device comprising: the light-emitting element accordingto claim 10, and a means controlling the light-emitting element.
 23. Alight-emitting element comprising: a first electrode, a secondelectrode, and an electroluminescence layer interposed between the firstelectrode and the second electrode, wherein the electroluminescencelayer comprises a light-emitting layer including a first light-emittinglayer and a second light-emitting layer over the first light-emittinglayer, wherein the first light-emitting layer contains a first organiccompound, a second organic compound, and a fluorescent substanceincluding a substance exhibiting delayed, fluorescence, wherein thesecond light-emitting layer contains a third organic compound and aphosphorescent substance, wherein a combination of the first organiccompound and the second organic compound forms a first exciplex, andwherein the first organic compound and the second organic compound aremixed in the first light-emitting layer to fot it a singlelight-emitting layer.
 24. The light-emitting element according to claim23, wherein the first light-emitting layer is in contact with the secondlight-emitting layer,
 25. The light-emitting element according to claim23, wherein a light emission spectrum of the first exciplex has a peakon a shorter wavelength side than a light emission spectrum of thephosphorescent substance.
 26. The light-emitting element according toclaim 23, wherein a difference in energy between a triplet excited leveland a singlet excited level of the first exciplex is greater than orequal to 0 eV and less than or equal to 0.2 eV.
 27. The light-emittingelement according to claim 23, wherein white light emission isexhibited.
 28. A light-emitting module comprising: the light-emittingelement according to claim 23, and a means controlling thelight-emitting element.
 29. A light-emitting device comprising: thelight-emitting element according to claim 23, and a means controllingthe light-emitting element.